This invention relates to distance-measuring equipment, and in particular, to interferometers.
A multi-axis interferometer generates several beams to be used in connection with distance measurement. To enhance the accuracy of distance measurements, these beams should all be perfectly parallel. One approach to ensuring parallel beams is to impose strict tolerances during the manufacture of the interferometer.
In practice, it is difficult to construct a multi-axis interferometer that generates perfectly parallel beams. As a result, it is often necessary to make minor adjustments to the beams that emerge from the interferometer. The extent and type of minor adjustments to be made varies between interferometers and also between different beams formed by the same interferometer.
One approach to correcting the beams that emerge from an interferometer is to incorporate an adjustable optical element, such a Risley prism, in the path of each beam emerging from the interferometer. The adjustable optical elements can then be individually adjusted to accommodate the imperfections in the particular beam with which it is associated. In particular, each individual prism is adjusted to alter the direction of propagation for its associated beam.
The invention is a corrector plate for any multi-beam interferometer or even more generally, a corrector plate for any multi-beam optical device for which beam pointing correction might be appropriate. For example, this invention uses an optical plate with a series of polished sub-apertures to correct the beam pointing of a multi-axis interferometer. The sub-apertures can be of any perimeter size, and have either a regular (circular or rectangular) or an irregular shape. An alternative embodiment will use a plate with a continuously varying curved surface rather than discreet sub-apertures. Furthermore, the beam pointing sub-apertures can be part of a separate plate that is added to the optical system (e.g., interferometer) or they can be formed in another optical component that is part of the optical system itself (i.e., not an add-on component).
In another aspect, the invention is a method of correcting for beam pointing errors by first measuring those errors and then forming multiple correcting sub-apertures in the plate and/or component.
We have identified the Magneto Rheological Finishing (MRF) process and machine as one appropriate way of making the sub-apertures described herein. A commercial machine that is available for fabricating the sub-apertures in accordance with that method is made by and available through OED Technologies of 1040 University Avenue, Rochester, N.Y. 14607. The method is described in the following patents: U.S. Pat. Nos. 5,971,835; 5,951,369; 5,525,249; 5,616,066; 5,795,212; 5,577,948; and 5,449,313; the contents of which are incorporated herein by reference.
It should be understood, however, that though we have identified the MRF process as a way of fabricating the sub-apertures, any other known methods for forming such sub-apertures by polishing or by any other known techniques could be used.
The invention provides a corrector plate that intercepts each of the beams that emerge from a multi-axis interferometer. Each beam illuminates a different aperture portion of the corrector plate. The aperture portion intercepted by a particular beam is configured to correct the characteristics of that particular beam.
In one embodiment, a monolithic corrector plate includes a substrate having an input face for intercepting a first beam emitted by the interferometer, and an output face opposite the input face. An aperture integral with the substrate is configured to transform a first beam intercepted by the input face into a second beam emerging from the output face.
In one aspect of the invention, the aperture is configured to form a second beam having a selected direction of propagation. The selected direction of propagation is different from a direction of propagation of the first beam. Preferably, the aperture is configured to form a second beam propagating in a direction orthogonal to the output face. One optical device for correcting the direction of propagation of the first beam is a prism.
In another aspect, the aperture is configured to form a second beam having a different phase front from the first beam. This can be achieved by providing an aperture that includes a lens. Preferably, the aperture is configured to form a second beam having a planar phase front, particularly one having a planar phase front parallel to the output face.
In another embodiment, the aperture merges continuously with neighboring apertures thereof. In this embodiment, the boundaries between apertures become less distinct.
In another embodiment, the invention provides an interferometer having an interferometer housing from which emerges a first beam. The interferometer further includes a substrate having an input face for intercepting the first beam from the interferometer, and an output face opposite the input face. An aperture integrated into the substrate is configured to transform the first beam incident on the input face into a second beam emerging from the output face.
In another embodiment, a multi-axis interferometer includes an optically transmissive monolith. The monolith has a multiplexer portion and a beam splitter portion. The multiplexer portion is configured to split an input beam into a corresponding plurality of intermediate beams, each of the intermediate beams being directed toward the beam splitter portion through a corresponding output port of the multiplexer portion. The beam splitter portion is configured to separate the intermediate beam into a measurement component and a reference component. An optically transmissive substrate is disposed to receive a first beam from the beam splitter portion. The first beam can include the measurement component or the reference component. The substrate includes an input face for intercepting the first beam, and an output face coupled to the input face. An aperture integrated into the substrate, is configured to transform the first beam incident into a second beam emerging from the output face.
These and other features of the invention will be apparent from the following detailed description and its accompanying figures, in which: